1. Field of the Invention
The present invention generally relates to a multi-beam laser scanning unit used in an image forming apparatus, such as a laser printer, and more particularly, to a sub-scanning interval adjusting apparatus of a multi-beam laser scanning unit adjusting a distance between at least two beams of the multi-beam laser scanning unit.
2. Description of the Related Art
Generally, a laser printer forms an image by processes of processes of focusing a laser beam from a laser scanning unit onto a photoreceptor drum in accordance with a video signal, developing an electrostatic latent image on the photoreceptor drum, and transferring the developed image onto a printing medium such as a paper. An exemplary structure of the laser scanning unit for use in the laser printer is shown in FIG. 1.
Referring to FIG. 1, the laser scanning unit is provided with a laser source 10, a collimating lens 11, a cylinder lens 12, a polygon mirror 13, an f-theta lens 15, and a reflective mirror 16.
The laser source 10 generates a laser beam 1, and usually a laser diode is used as the laser source.
The collimating lens 11 transforms the laser beam 1 from the laser diode 10 into a parallel or convergence ray of light (laser beam 1 having parallel rays) with respect to an optical axis.
The cylinder lens 12 focuses the laser beam 1 passed through the collimating lens 11 onto a surface of the polygon mirror 13 in a form of a line in a horizontal direction.
The polygon mirror 13 is rotated by a motor 14 at a constant speed to perform scanning by displacing the laser beam 1 passed through the cylinder lens 12 with a constant linear velocity in the horizontal direction.
The f-theta lens 15 has a predetermined refractivity with respect to the optical axis, deflects the laser beam 1 reflected from the polygon mirror 13 at constant velocity towards the main scanning direction (arrow A), and compensates for aberrations, thereby focusing the laser beam 1 on the scanning surface.
The reflective mirror 16 reflects the laser beam 1 passed through the f-theta lens 15 towards a certain direction, thereby focusing the laser beam 1 on a surface of a photoreceptor drum 17 in a form of a dot.
Further provided is an optical sensor 19 for horizontal-synchronizing the laser beam 1. The optical sensor 19 is provided with a synchronization signal detecting reflective mirror 18, which reflects the light passed through the f-theta lens 15 towards the optical sensor 19.
The above-mentioned components are assembled in a single frame (not shown) to form the laser scanning unit. Further, a transparent glass member (not shown) is attached to an outer side of the frame to prevent foreign substances, such as dust, from being introduced into the laser scanning unit.
Increasingly, demands on the laser printer that prints at a higher printing speed have been growing. In order to increase the printing speed of the laser printer, a scanning speed of the polygon mirror 13 needs to be increased. And in order to increase the scanning speed, an R.P.M of the motor 11 has been increased to increase the R.P.M of the polygon mirror 13. This way, however, has been accompanied by problems, such as an increase of noise, and also a high material cost mainly due to the requirement for a high-speed motor.
In an attempt to solve the above-mentioned problems, recently employed is a multi-beam laser scanning unit that has a plurality of light sources. In this case, if the multi-beam laser scanning unit having two light sources is employed to guarantee the constant printing speed, the motor 14 for the polygon mirror 13 has a half R.P.M of a general motor.
With the multi-beam laser scanning unit, two lines of a given resolution are simultaneously formed on the photoreceptor drum 17 in a perpendicular direction (hereinafter called ‘sub-scanning direction’) in relation to the main scanning direction.
In the meantime, when components of the laser scanning unit are assembled, errors inevitably occur due to assembling status or precision of the respective components.
Accordingly, a space interval between two lines in the sub-scanning direction is deviated from a normal interval that is predetermined for the corresponding resolution. And this usually causes an undesired phenomenon, so-called ‘jittering’, in the laser printer with high-resolution, deteriorating a sharpness of a printing image.
In order to solve the above-mentioned problems, as shown in FIG. 2, an apparatus 20 is provided to adjust an interval between multi-beams in the sub-scanning direction.
Referring to FIG. 2, the multi-beam interval adjusting apparatus 20 includes a substrate 21 and a sub-assembling plate 23.
The substrate 21 is mounted with a laser diode 22 as a light source, and a circuit driving the laser diode 22. The sub-assembling plate 23 has a lens barrel 23-1 formed at a center thereof, and a plurality of elliptical holes 23-3 allowing the sub-assembling plate 23 to rotate about the lens barrel 23-1 by a predetermined angle. Assembled in the lens barrel 23-1 is a collimating lens 24. A tap hole 23-2 is formed on the sub-assembling plate 23 to secure the substrate 21 on the assembling plate 23. The substrate 21 is assembled with respect to the sub-assembling plate 23 by screws 26 through screw holes 21-1. Then, after the lens barrel 23-1 of the sub-assembling plate 23 is inserted in a hole 25-1 formed on the frame 25, the sub-assembling plate 23 is fastened to the frame 25 with screws 27. The screws 27 are fastened to the frame 25 through the elliptical holes 23-3 of the sub-assembling plate 23 and screw holes 25-2 of the frame 25. Accordingly, the lens barrel 23-1 of the sub-assembling plate 23 is adjustable in a precise position with respect to the hole 25-1 of the frame 25.
Accordingly, when laser light (a laser beam) is emitted from the laser diode 22, the laser beam passes through the lens barrel 23-1 and is incident on the collimating lens 24 and concentrated thereon in a form of a parallel ray.
In a case that the two lines of the laser beam concentrated on the photoreceptor drum 17 in the sub-scanning direction are disposed at an interval while being deviated from a predetermined interval, such deviation is adjusted by minutely moving and fastening the sub-assembling plate 23 on the frame using the screws 27.
However, the multi-beam interval adjusting apparatus 20 as described above has a shortcoming. That is, the image formation of the two laser lines in the scanning direction starts from different positions. By way of one example, FIG. 3A shows two laser diodes 22-1, 22-2 that are provided on the same line and in perpendicular relation with respect to the scanning direction, and at a predetermined interval (p). As are predetermined, the two lines are formed on the photoreceptor drum 17 at the predetermined interval (p). Then when the two lines are formed at an interval other than the predetermined interval (p), such deviation is adjusted by adjusting the interval between the laser diodes. Accordingly, the sub-assembling plate 23 is rotated to adjust the interval between the two lines as predetermined. However, while the interval between the two laser diodes 22-1, 22-2 in the sub-scanning direction becomes p′, the laser diodes 22-1, 22-2 are deviated from a predetermined plane in the main scanning direction by a distance (Δ) (see FIG. 3B). As a result, the image formation in the scanning direction starts differently. Such a problem can be solved by employing a software or device for compensation.
However, employment of the compensation software or device causes a structure of the laser scanning unit to be complex, and a material cost increases.